The rotary drilling of a borehole is accomplished by rotating a drill string having a drill pipe and a drill bit at its lower end. Weight is applied to the drill bit while rotating to create a borehole into the earth. The drill string is hollow and sections are added to the drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of the strata must be removed from the borehole and the drill bit must be cooled to extend its useful life. Both of these functions are accomplished by circulating a fluid down through the drill string and up to the surface between the drill string and the wall of the borehole. Generally, the borehole into which the casing or liner is introduced is filled with drilling mud.
Most drilling muds have either an aqueous or an oil base. Water base drilling fluids are not only popular, but often are necessary in order to meet applicable environmental regulations. The current generation of water-based drilling fluids often contains a high concentration of additives to improve borehole stability, improve lubricity, and to control seepage loss to the formation. A high concentration of additives, such as glycols, silicates, or surfactant-lubricant blends, makes it difficult to formulate some higher density fluids (esp. silicate fluids) to balance desired Theological properties with adequate shale stabilization and filtration control.
Silicate drilling fluids are capable of giving superior borehole stability. The soluble silicate in a silicate drilling fluid is believed to invade the shale matrix and either (a) the soluble silicates precipitate when they contact the Ca.sup.2+ and Mg.sup.2+ available in the reservoir fluid, or (b) the reduced pH encountered in the reservoir fluid causes polymerization to occur. Both mechanisms are capable of effectively plugging the pore throats of the shale and reducing filtrate invasion.
A principal requirement of any good drilling fluid, including a silicate drilling fluid, is that the fluid remain pumpable while suspending a sufficient amount of weighting additives to meet the desired density requirements, particularly with respect to preventing gasification and blow-outs. One commonly used weighting agent is barite. Unfortunately, the use of high concentrations of barite in silicate drilling fluids results in significant depletion of silicate during exposure to heat. Silicate depletion may diminish borehole stability using silicate drilling fluids.
Alternative formulations are needed in which the silicates are not depleted during exposure to heat, and which have desirable rheology, shale stabilization, and filtration control properties.